Stabilized chemically reactive heatsensitive copy-sheet



April 3, 1962 E. A. GRANT, JR 3,028,254

STABILIZED CHEMICALLY REACTVE HEAT-SENSITIVE COPY-'SHEET Filed Sept. 2l, 1959 bac/nhg T Toffe/Wr )f5 United States This invention relates to heat-sensitive copy-sheets useful in the thermographic reproduction of graphic originals.

Heat-sensitive copy-sheets are known which undergo chemical reaction and are visibly changed when briey heated as in the thermographic process. One such copysheet which has `attained considerable commercial success has a heat-sensitive layer containing as reactants a noble metal soap, such `as silver behenate, and an aromatic organic reducing agent, for example protocatechuic acid. The initial sheet is white or near-white in appearance, and is converted to black or brownish black during thermographic copying or when momentarily pressed against a metal test bar heated to a conversion temperature within the approximate range 90-150 C. The sheet is stable under normal use and storage conditions, being neither discolored nor rendered insensitive to heat when stored, used in copying, or permitted to remain exposed on desk-tops or the like under normal oce conditions. It has been found, however, that moderate exposure to direct sunlight causes darkening of the initially white sheet. For example, an initially white sheet of the composition above-identitied was found to be converted to a light buff or golden brown color when exposed for several days to direct sunlight through an oce window. Although the sheet remained heat-sensitive and capable of use in thermographic copying of printed originals, the resulting copy was lower in contrast than was a copy prepared with an unexposed white sheet.

I have now found that heat-sensitive copy-sheets of the type just described may be stabilized against the darkening or discoloring effects of direct sunlight. There is made available a heat-sensitive copy-paper which is initially White or nearly White, which does not observably discolor on prolonged exposure to direct as Well as ditfuse sunlight either before or after heat-copying, and which remains fully heat-sensitive and capable of providing high-contrast reproductions of graphic originals by thermo-copying.

These and other advantages are attained, in accordance with my invention, by incorporating in the heat-sensitive coating signicant small amounts of organic compounds containing activated 1,2-alkylene imine rings having the structure Awherein each of R' and R" represents a hydrocarbon radical or hydrogen, said ring being capable of ringopening reactions with substitution of a hydrogen -atom on the nitrogen atom.

Certain N,Nbis1,2alkylene amides, carbamates, and other allied compounds have in particular been found t possess the required `chemical properties; and in addition these difunctional materials are available as normally stable liquid or solid compounds easily incorporated in the liquid coating compositions, they are essentially colorless or of such low hiding power as to impart no observable color to the heat-sensitive coating, and they provide fully effective stabilization of the coating at surprisingly low concentrations. Although my invention is atent O 3,628,254 Patented Apr. 3, 1962 not to be considered as restricted thereto, these classes of polyfunctional ethylene imines are greatly preferred by me in the practice thereof. Analogous monofunctional or higher polyfunctional compounds are likewise useful but the difunctional materials are readily obtainable and are preferred.

One suitable difunctional 1,2-alkylene imine is N,N bis-l,Z-ethylenisosebacamide, prepared, for example, by dropwise addition of a solution of 95.6 parts by weight of isosebacoyl dichloride in 400 parts of diethyl ether to a solution of parts of potassium carbonate and 43 parts of ethylenimine in 800 parts of Water, with vigorous stirring while maintaining the temperature below 15 C. The ether layer is separated, dried over sodium hydroxide in the cold, ltered, and the ether removed under reduced pressure. The product remains as a water-white liquid. The isosebacoyl dichloride employed is prepared from commercially available isosebacic acid which has been shown to consist of 72-80% of Z-ethylsuberic acid, l2l8% of 2,5-diethyladipic acid, and 6-10% of nsebacic acid.

Another useful difunctional 1,2-a1kylene imine is N,N bis l,2-ethylene(1,lisopropylidenebis(p-phenylenoxy) di-2-propanol)carbamate, which is similarly prepared by condensation reaction at low temperature between ethylenimine and l,1'-isopropylidenebis(p-phenylenoxy)di-2- propanol-bis-chlorocarbonate in solution and in the presence of an acid acceptor.

Analogous useful difunctional alkyl-ene imines include: N,N' bis 1,2-ethylensebacamide; N,N'bis-1,2ethylen suberamide; N,Nbis1,2-propylenazelamide; N,Nbis 1,.Z-ethylenoctadecanoyl-dicarboxylic acid amide; N,N bis-2-methy1-1,2-butylensebacamide; N,N-bisl,2ethylen isophthalamide; N,N'bisl,2-ethylene( 1,4-butanediol) carbamate; N,Nbis1,2-butylene-(triethyleneglycol)-carbamate; N,Nbis1,2ethy1ene(polybutyleneglycol)-carbamate; and N,N'bis1,2ethylene(1,1isopropylidene bis-(p-cyclohexanol) )-carbamate. In general, these compounds will be seen to contain ethylene imine radicals having the structure and which are rendered active by being bonded to an electron-withdrawing group, for example an acyl or oxyacyl group. More particularly, the preferred difunctional alkylene imines illustrated by the compounds hereinabove listed may be represented by the structural wherein R and R" are as previously indicated and R is a divalent organic radical.

In accordance with the principles of the present invention, significant small amounts of the alkylene imine, for example of the order of one-tenth to one percent by weight of the visibly heat-sensitive reactants, are added to the heat-sensitive composition just prior to its application in liquid form as a thin uniform layer on a suitable paper or other carrier web, as will now be more fullir described in the following illustrative but non-limitative examples and the appended drawing.

Example I A prior art heat-sensitive copy-sheet is first prepared as a control, as follows:

A dispersion of silver soap is first prepared by uniformly mixing together in a ball mill'9.9 parts by weight of silver behenate, 2.5 parts of polystyrene resin, 1.6 parts of phthalazinone and 86 parts of methylethyl ketone. Separately, a solution is prepared rof 2.44 parts of spiroindane, 0.24 part of tetrachlorophthalic anhydride, 0.24 part of 4-behenoyl pyrogallol, 9.8 parts of polystyrene, and 4.9 parts of polymethylstyrene in suicientof a mixture of equal parts of acetone and commercial heptane to make 100 parts. 'Ien parts of lthedispersion 'and thirteen parts of the solution are mixed together,'form ing a coatable smooth mixture vwhich is applied to thin transparent paper, in this instance 30 lb. rnap overlay tracing paper, and dried at normal room temperature. Application is by means of a coating knife or bar at an orifice of about l3 mils. The sheet is visibly heat-sensitive and is useful in the thermographic reproduction process. When the sheet is momentarily heated, an oxidation-reduction reaction occurs between thesilver soap and the spiroindane phenolic reducing agent, with liberation of silver and visible darkening of the heat-sensitive layer. The binder resins unify the sheet and to some extent protect the active components. The remaining minor components assist in improving the image color or in other ways but are not essential for the purposes of the present invention and may be omitted.

A further coating is next applied of a mixture of 32 parts of titanium dioxide pigment, v5 parts of water-insoluble starch, and 6 parts of styrene-isobutylene copolymer resinous binder in 57 parts of heptane, ground in a ball mill until smooth, and in just sufiicient thickness lto provide an opaque thin protective surface coating. The dried sheet is white in appearance. It converts to-a Ydark brownish yblack on brief contact withy a metal test rod heated to 1'50" C. an'd in contact with a differentially radiation-absorptive intensely Yirradiated graphic original in the thermographic reproduction process. The copysheet is stable under usual oice storage and use conditions, -but under exposure to direct sunlight is found to discolor badly, being converted to a tan or light brown color when placed in a west-facing window and subjected to direct sunlight exposure for one day. The colored sheet is heat-sensitive and produces visibly distinct imageforming areas in the thermographic copying process, but the contrast between image and background areas is much reduced as compared with copy provided by sheets not so exposed.

Additional heat-sensitive copy-sheets are prepared using the same silver soap and reducing agent compositions except for the addition thereto, just prior to final mixing, of varying proportions of N,Nbis1,2ethylene isophthalamide, the amounts added being respectively 0.3, 0.6, and 1.0 part by weight in 100 parts of the solution containing the spiroindane reducing agent. Each sheet as initially prepared is found to be substantially identical with the control sheet in converting from white to brownish black on heating with the test bar or in thermocopying. In direct sunlight, the sheet prepared with 0.3 part of the ethylene imine shows no observable darkening for about one week; at 0.6 part, observable darkening is first noted in about two weeks; and at 1.0 part, about four weeks is required to produce observable darkening. Each of the improved sheets is found to be equally heat-sensitive after the exposure test, producing fully equivalent thermographic copies.

At the concentrations listed and with the specific reactants as defined in the foregoing example, it may be shown that the proportions of stabilizer component are respectively 0.3%, 0.6%, and 1.0%v based on the total of silver soap and spiroindane reducing agent. 'Ihe proportions of the major reactants are on the order of one part of spiroindane to three parts of silver behcnate, a proportion which has been found to produce optimum results in terms of copy-sheet performance. It will be apparent that other specifically different metal soaps and organic reducing agents may require specifically different ratios `for optimum performance and that some further variation in the relative amount of stabilizer may then be necessary. Other stabilizers, particularly those of greatly dissimilar molecular weight or functional equivalency, may likewise require to be used in proportions differing appreciably from those herein illustrated in order to attain equivalent stabilization. In many instances as little as one-tenth percent of ethyleneimine stabilizer is found t0 produce adequate resistance to sunlight; in others it may be found desirable to employ one percent or somewhat larger amounts.

Example 2 The preparative and test procedures .of Example 1 are repeated, in this instance with the substitution for the N,Nbis1,Z-ethyleneisophthalamide of 0.3 part of the compound N,N'bis1,2ethylene(1,1-isopropylidenebis (p-phenylenoxy)-di-2-propanol)-carbamate. After one week in direct sunlight the sheet exhibits a very faint tan discoloration. At higher proportions of the stabilizer the amount of discoloration is reduced, none being observable at 1.0 percent. Full heat-sensitivity in the thermographic process is retained.

Example 3 The compound N,Nbis1,Z-ethyleneisosebacamide is substituted as the 'stabilizer component in the formulation of Examples l and 2. At 0.3 percent of the compound the sheet exh-ibits no observable discoloration in one week under direct sunlight.

Although no entirely adequate explanation of the advantageous results here obtained has yet been made apparent, it is believed that the activated ethylene imine radicals act, during sunlight exposure of the sheet, as electron-acceptors or scavengers, presumably by reaction involving opening of the activated imine ring. Surprisingly, however, such action appears to have no adverse effect on the color-producing oxidation reduction reaction occurring in the heat-sensitive layer under the infiuence of high temperatures in thermocopying.

As ordinarily prepared, the stabilizer materials here employed are found to contain traces of'chlorides. When the stabilizers are added in relatively large amounts to compositions containing silver soaps, these impurities react with the silver to form light-sensitive silver chloride which itself then causes darkening under the influence of sunlight. Where large amounts of stabilizer are to be added, it is therefore necessary that the material be highly purified. Ordinarily, however, amounts corresponding to about one percent or less of the stabilizer based ,0n the total weight of the dry constituents of the heat-sensitive layer are found to provide fully adequate sunlightstability. In these small amounts the preferred di-imine stabilizers are found to be fully soluble and compatible in the solvent mixtures employed, thereby ensuring even distribution and maximum effectiveness of the material.

The stabilizer component maybe added directly to the pre-mixed liquid coating composition, or may be added to either of the soap dispersion or the solution of reducing agent prior to combining the two for coating. It is ordinarily preferred to add the stabilizer to the soap dispersion, since the resulting mixture is stable under prolonged storage whereas the stabilizer has in some instances been found to react slowly with one or more cornponents of the solution of reducing agent, causing darkening of the solution and reducing the heat-sensitivity of the resulting coated sheet. It is lalso found desirable to keep separate the silver soap dispersion and the solution of reducing agent until just prior to use.

It will be appreciated that soaps of other organic acids and other noble metals, and various other reducing agents, may be substituted for the specific reactants here-- apnea-354 in provided. Likewise, other resinous or polymeric binders, and solvents or solvent mixtures appropriate thereto, may be utilized. The reactants may alternatively be carried within a fibrous web, eg. paper, in the substantial absence of a binder component; or a hlm-forming binder may itself serve as a carrier web. Various addition-al additives such as pigments, fillers, plasticizers or the like may be introduced if desired. These and other modifications are well known in the art and form no part of the present invention.

There iS thus provided a heat-sensitive copy-sheet having a heat-sensitive layer including an organic acid soap of a noble metal and yan organic reducing agent reactive therewith, and capable of greatly improved resistance to darkening under exposure to direct sunlight by reason of the incorporation in said heat-sensitive layer of a signicant small proportion of a stabilizer compound containing an activated ethylene imine radical.

What is claimed is as follows:

1. A sta-bilized chemically reactive heat-sensitive copysheet which remains visibly heat-sensitive and does not observably discolor on prolonged exposure to Sunlight, said copy-sheet including a heat-sensitive layer comprising a normally visibly stable chemically reactive heatsensitive mixture of an organic acid soap of a noble metal and an organic reducing agent for the noble metal ions, and a signicant small proportion of a stabilizer component having an activated ethylene imine radical bonded to an electron-withdrawing group of the class consisting of Iacyl and oxyacyl groups, said radical having the structure CR'R" wherein R and R are each selected from the class consisting of hydrogen and lower alkyl radicals.

2. A stabilized chemically reactive white heat-sensitive copy-sheet which remains visibly heat-sensitive and does not observably discolor on prolonged exposure to sunlight, said copy-sheet including a heat-sensitive layer comprising a normally visibly stable chemically reactive heat-sensitive mixture of a silver soap of an organic acid and an aromatic organic reducing agent for the silver ions, and'a signieant small proportion of a polyfunctional stabilizer component having 'a plurality of activated ethylene imine radicals each bonded to an electron-withdrawing group of the class consisting of acyl and oxyacyl groups, and having the structure CRIB/l wherein R and R" are each selected from the class consisting of hydrogen and lower alkyl radicals.

3. A stabilized chemically reactive heat-sensitive copysheet which remains visibly heat-sensitive and does not observably discolor on prolonged exposure to sunlight, said copy-sheet including a heat-sensitive layer comprising a normally visibly stable chemically reactive heatsensitive mixture of a water-insoluble normally solid silver salt of an organic acid and an aromatic organic reducing agent for the silver ions, a resinous binder, and about 0.1-1.0 percent, based on the total weight of said salt and said reducing agent, of N,N-bis-l,2':lkylene amide.

4. A stabilized chemically reactive heat-sensitive copysheet which remains visibly heat-sensitive and docs not observably discolor on prolonged exposure to sunlight, said copy-sheet including a heat-sensitive layer comprising a normally visibly stable chemically reactive heatsensitive mixture of a water-insoluble normally soiid silver salt of an organic acid and an aromatic organic reducing agent for the silver ions, a resinous binder, and about 0.1-1.0 percent, based on the total weight of said salt and said reducing agent, of an N,N-bis1,2alkylene carbarnate.

5. The method of improving the resistance to darkening under direct sunlight exposure of a heat-sensitive copy-sheet having a heat-sensitive layer containing -a normally visibly stable chemically reactive heat-sensitive mixture of a noble metal soap of an organic acid and an organic reducing agent for the noble metal ions as major image-forming reactants, comprising uniformly incorporating with said reactants in said layer a significant small proportion of a stabilizer component having at least one activated alkylene imine radical bonded to an electronwithdrawing group of the class consisting of acyl and oxyacyl groups and having the structure wherein R and R are each selected from the class consisting of hydrogen and lower alkyl radicals.

6. The method of improving the resistance to darkening under direct sunlight exposure of a heat-sensitive copy-sheet having a heat-sensitive layer containing a normally visibly stable chemically reactive heat-sensitive mixture of a noble metal soap of an organic acid and an organic reducing agent for the noble metal ions as major inzage-forming reactants, comprising uniformly incorporating with said reactants in said layer a signiiicant small proportion of an N,N-bis-1,2-alkylene amide as a stabilizer component.

7. The method of improvinn the resistance to darkening under direct sunlight exposure of a heat-sensitive copy-sheet having a heat-sensitive layer containing a normally visibly stable chemically reactive heat-sensitive mixture of a noble metal soap of an organic acid and an organic reducing agent for the noble metal ions as major image-forming reactants, comprising uniformly incorporating with said reactants in said layer a significant small proportion of an N,N'-bis-l,2alkylene carbamate as a stabilizer component.

Miller et al. Dec. 22, 1953 Owen Oct. 27, 1959 

1. A STABILIZED CHEMICALLY REACTIVE HEAT-SENSITIVE COPYSHEET WHICH REMAINS VISIBLY HEAT-SENSITIVE AND DOES NOT OBSERVABLY DISCOLOR ON PROLONGED EXPOSURE TO SUNLIGHT SAID COPY-SHEET INCLUDING A HEAT-SENTIVE LAYER COMPRISING A NORMALLY VISIBLY STABLE CHEMICALLY REACTIVE HEATSENSITIVE MIXTURE OF AN ORGANIC ACID SOAP OF A NOBLE METAL AND AN ORGANIC REDUCING AGENT FOR THE NOBLE METAL IONS, AND A SIGNIFICIENT SMALL PROPORTION OF A STABILIZER COMPONENT HAVING AN ACTIVATED ETHYLENE IMINE RADICAL BONDED TO AN ELECTRON-WITHDRAWING GROUP OF THE CLASS CONSISTING OF ACYL AND OXYACYL GROUPS, SAID RADICAL HAVING THE STRUCTURE 